It was an old bash 2.x which was still GPLv2. Zsh being MIT licensed posed no licensing issues for inclusion into MacOS. I switched to zsh on Snow Leopard and a

It was an old bash 2.x which was still GPLv2. Zsh being MIT licensed posed no licensing issues for inclusion into MacOS. I switched to zsh on Snow Leopard and above and never looked back. Didn't use the fancy stuff much and still use bash on Linux.